One of the problems with making a high speed logic analyzer probe is that the probe will perturb the system under test. These types of probes generally have input impedances of a few hundred ohms, but there is usually a short compliant contact between the electrical contact on the circuit board of the device under test (DUT) and the input resistance, which is described in U.S. Pat. Nos. 6,888,361 and 6,781,391, which are incorporated herein by reference. In multi-conductor probing applications involving logic analyzer probes or Mixed Signal Oscilloscope probes, an interconnect is employed to achieve individual compliance of each contact being tested, resulting in a stub.
A prime example is a probe head with pogo pins. These provide adequate compliance, but become an unterminated stub on the circuit being tested. This unterminated stub will reflect energy back into the system under test, so a long time delay in the compliant contact is undesirable. Electrical stubs can impact the circuit being probed, so probe designers seek to minimize the stubs. The ideal probe tip would have a resistive element at the point of contact, thereby creating no stub.
However, existing probing methods place input resistors approximately 0.100 inches from the DUT contact points of the circuit to be probed, creating a ‘stub’ on the probed circuit. Two elements contribute to the ‘stub length’: (1) the compliant interconnect socket, and (2) limitations of placing resistors close to the edge of the circuit board of the probe head. The length of the time delay in the compliant contact is one of the factors that limit the speed at which the probe can be used.
An example of a compliant interconnect socket is a Spirit® Compression Test Probe, manufactured and sold by Samtec, Inc., New Albany, Ind. The Spirit® Compression Test Probe has mounting brackets positioned on either side of parallel rows of contact points on a circuit board of the DUT. Parallel rows of passive resistors corresponding to the parallel rows of contact points on the circuit board are embedded in a connector housing that is positioned between the mounting brackets. Each passive resistor is secured to a lower spring contact and an upper spring contact with the lower spring contact engaging one of the contact points on the circuit board and the upper spring contact engaging a corresponding contact point in a probe head. The stub length of the lower spring contact is 0.100 inches.
In previous implementations of logic analyzer probes, the resistor has resided on a circuit board in the probe head, and the compliant contact comes into contact with the edge of the circuit board. The length of the contact and the amount the resistor is set back from the edge of the circuit board both contribute to the unterminated stub length. The distance that the resistor is set back from the edge of the circuit board is determined by circuit board manufacturing constraints. In addition, the edge wrap on the circuit board produces an edge that is somewhat rough and departs from planarity because of the process of routing and plating the laminated circuit board edge. Therefore, there is a need for a resistive probing tip system that enables probing at higher speeds by decreasing the stub length on the probed circuit.